Photoelectrochemical (PEC) devices are among the promising systems to convert and store solar into chemical energy in the form of electrochemical fuels, such as hydrogen and other organic chemicals.[1] Solar redox flow cells (SRFCs) have emerged with the idea of using PEC cells for charging two electrochemical fuels, posilyte and negalyte. To reach the highest energy conversion efficiencies, for each semiconductor employed in the SRFC two redox couples should match with their Nernst redox potentials. Therefore, the semiconductor-electrolyte interface plays a crucial role in the performance of a SRFC, so semiconductor must be abundant, cheap, efficient and stable.[2]

Tantalum nitride (Ta3N5) shows a narrow bandgap (2.1 eV)[3] and a maximum theoretical photocurrent-density of 12.9 mA⸳cm-2. In few years of research, Ta3N5 reached the maximum efficiency of 12.1 mA⸳cm-2 for opaque metallic samples, demonstrating its potential contrasting with other studied materials, such as hematite[4]. However, opacity reveals an issue for tandem systems. Semitransparent photoelectrodes can overcome this problem and has been receiving special attention. Domen[5] was the first to develop Ta3N5 photoelectrodes by electrophoretic deposition (EPD) coated with an IrO2 overlayer for PEC water splitting, showing a current density of 2.0 mA∙cm2 at 0.2 VAg/AgCl using a 0.1 M Na2SO4 solution as electrolyte.

This work has also focused on the EPD technique. The effect of varying the time and the applied potential of deposition on the Ta3N5 film thickness and then on its performance was studied. Then, a treatment with TaCl5 methanolic solution has been employed [5, 6] for improving the adhesion of the film particles. A TaOx:TiO2 (TTO) underlayer was also employed by ALD as to improve Ta3N5 film adhesion into FTO and electron transport at different concentrations concentrations. At the end, the samples were annealed in an ammonia reducing atmosphere (with different flowrates) to promote semiconductor adhesion to the substrate. This work aims at optimizing Ta3N5 photoelectrodes employing adesign of experiments approach. A response surface methodology was applied to factors: (i) TTO ratios; (ii) deposition time; (iii) applied potential; (iv) annealing temperature. The best results were obtained to the conditions of TTO 0.014; 3 min deposition time; 12.5 V applied potential; 525 ˚C annealing temperature, obtaining 4.01 mA⸳cm-2 at at 0.2 VAg/AgCl, without dopants or co-catalysts.